Anionic dyes, such as fiber reactive dyes and direct dyes, are currently employed for dyeing cellulose fibers because of their wide shade range, ease of application, and adequate wet fastness properties for many end uses.
There are, however, certain environmental problems related to the utilization of such dyes, which occur because high amounts of electrolyte and alkalinity must be used, and the relatively poor uptake of such dyes into the cellulosic fibers. Depending on the application method, shade depth and dye type, only 70-80% of the dye becomes attached to the substrate using conventional dyeing methods. Consequently, dyehouse effluents contain an unacceptably high level of unfixed dye, electrolytes and alkaline residues which can cause environmental hazards, and compliance problems with EPA discharge standards.
The above-described problems were addressed in part by Weltrowski et al in U.S. Pat. No. 5,501,711 by mildly oxidizing the fibers, subjecting the oxidized fibers to reduction with a solution of chitosan oligomers, stabilizing the chitosan-treated fibers by addition of a reducing agent such as dimethylol dihydroxyethyleneurea (DMDHEU), and then dyeing the thusly treated fiber. This process involves 4-5 steps, and even then does not address the problems of dye fastness and the high cost of the chitin treatment. This process represents a substantial improvement in dye pickup, and therefore improved dye exhaust from the dye bath. However, the discharge of metals into dye bath effluents remains a particularly troublesome problem, because so many dyes contain substantial quantities of metals. For example, many blue dyes contain copper, and many brown dyes contain chromium. In addition, some dyes contain such metals as cobalt and magnesium. Traces of catalyst are a still further source of metallic contaminants from dye bath effluents. Furthermore, certain analogs, such as N-3-chloro-2-hydroxypropyltrimethylammonium chloride, may produce toxic amounts of epichlorohydrin when they are contained in alkaline solutions. Therefore, still further improvements are needed to improve dye uptakes, to reduce electrolyte concentrations in the dye bath, and to reduce the quantity and toxicity of the discharge from the dye bath, in each case without sacrificing the dye ability of the cellulose fiber.